Elastomeric and plastic sheet material is formed by various methods including calendering, extrusion, drawing and the like. Due to the capital expense of the forming apparatus, the original width of plastic sheeting is relatively limited. Calender rolls or extrusion heads suitable to form for example 100 foot wide sheets of plastic materials are prohibitively expensive.
Calender rolls are typically 5 feet in width or less, although, a very few 10 foot wide calender rolls are now being used. This generally limits the width of formed sheet material, particularly formed rubber sheet material, to 5 feet. Many applications require substantially wider material. Roofing applications typically require 50 to 100 foot wide rolls.
A method of making very wide rubber sheeting is disclosed in Hollis U.S. Pat. No. 4,337,112. This patent discloses an apparatus for making an indefinite length composite sheet of very large predetermined width from indefinite length sheet stock having a relatively narrow width. According to this method, 5 foot wide sheet stock is rolled out, for example to 50 feet, and bonded to a second sheet along a horizontal edge to from a 50.times.10 foot wide sheet. This formed sheet is then advanced and another 50 foot long section is unwound and bonded to the 50.times.10 foot wide sheet to form a 50.times.15 foot sheet. This continues thereby forming a 50 feet wide sheet which is as long as desired. The sheet is then rolled up, cured and transported for use. Since the seams are parallel with the roll centerline, there is no problem with build up of material anywhere along the roll. This method is most efficient for particularly wide rolls, but becomes less and less advantageous as narrower width rolls such as 10 foot wide rolls are produced.
To form a sheet with a 10 foot width, a more economical method is to continuously, longitudinally join two 5 foot wide sheets forming a 10 foot wide sheet with a longitudinal seam down its centerline. Such a longitudinal seam is inherently preferred because there are fewer seams in a given sheet providing less waste and fewer cross seams for a roof installer to contend with. These cross seams represent an inherent weak point in all roof installations and often are the source of early failure. Longitudinal splices are also preferred because machinery can control seam overlap more accurately than with horizontal splicing.
Typically, a splice has a cross section as shown in FIG. 1A. The important thing to note with this seam is that it is twice the thickness of the normal sheet material. When such a sheet is rolled up, the double thickness of material creates a bulge in the roll.
An alternate embodiment of this seam is shown in FIG. 1B wherein two sheets are formed each having a stepped portion along opposed edges. These edges are joined to form a seam but the stepped portions do not overlap. The purpose of this seam is simply to reduce excess rubber, and therefore excess material cost in the seam area. This reduces the quantity of the material needed for such a wide seam, but still this prior art seam employs a double thickness of material and does not eliminate the problem of the bulge.
If sheets are longitudinally spliced using these well known splice constructions and an attempt is made to roll the sheet on round cores used to store and transport the sheet, an unacceptable bulge of material rapidly builds up at the center in the vacinity of the splice due to its double thickness thus making storage and transport impractical. U.S. Pat. No. 4,379,114 discloses a sheet bonding seam wherein two sheets with stepped portions are bonded together. As shown in FIG. 9, which is only briefly described in the patent, the half thick portion of one edge overlaps a full thick portion of another edge. However, the planar portion of the first edge does not bond to the planar portion of the second edge. This provides for a very uneven seam which is likely to fail or include pinholes. Accordingly, it is unsuitable for use in applications requiring a waterproof membrane, such as in roofing applications. The seam also has a thickness in excess of twice the thickness of the formed sheet material. Other references that disclose bonding sheet material together include Stearns U.S. Pat. No. 219,182 relating to a carpet lining, Ecureux U.S. Pat. No. 3,619,314 relating to a reinforced elastomeric sheet, and German Offen. 1,920,480 apparently relating to conveyor belts.
In roofing applications seams are formed from sheets with stepped edges. The edges are stepped to save material. There is no concerns with the thickness of the seam. Therefore, the seam includes areas where a full thickness portion of one edge overlaps a full thickness portion of a second edge.